This invention relates to a radiant oven and is more particularly concerned with a high efficiency convection stabilized radiant oven and a process of drying coated objects.
Infrared energy has been used for years as a form of energy to cure or dry coatings. The fuel source has usually been electricity or gas. In most designs of infrared ovens, gas burners or electric elements were usually used to produce the infrared radiation. These burners or electric elements usually operated in a temperature range from 1200.degree. F. to 3000.degree. F. A typical gas fired infrared burner of this type is described in my U.S. Pat. No. 3,277,948 (Radiant burner utilizing flame quenching phenomena). Because of the high energy levels generated at these temperatures, the burner surface area (radiating emitting surface) was usually small compared to the total area of the processed parts or material. Usually reflective material was mounted between the burners or electric elements to reflect the radiant energy which was not absorbed by the processed parts or material being dried or cured. As the reflectors aged and became soiled their reflective qualities decreased and the oven efficiency rapidly decreased.
In the past I have developed the HIGH HEAT TRANSFER OVEN of U.S. Pat. No. 4,235,023 which generates high turbulence adjacent to the painted or coated objects being dried by using spaced overhead fans dispersed along a tunnel oven so that air is directed in a turbulent condition against successive objects moved beneath successive fans. A part of the air which returns to each fan is reheated. Thereafter, I developed the RADIANT WALL OVEN AND PROCESS OF DRYING COATED OBJECTS of U.S. Pat. No. 4,546,553 in which opposed curved walls direct infrared radiant heat against successive coated or painted objects passed through an oven chamber, the walls being heated by turbulent air directed by fans against the back sides of these curved walls. The heated air thereafter was passed along the inside surfaces of the curved walls and a blower withdrew the air from the chamber. The primary drying achieved in the oven of U.S. Pat. No. 4,235,023 was through convection heating while the primary drying achieved in the oven of U.S. Pat. No. 4,546,553 was through radiant heating.
My prior art Radiant-Wall Oven used individual curved walls coated with a high emissivity coating which produced a shape factor very nearly equal to 1. Depending upon the emitter temperature, heat was transferred to the coated or painted object such as a vehicle body at almost any rate desired.
My ovens of U.S. Pat. Nos. 4,235,023 and 4,426,792 are used in many applications, especially where coated metal parts are involved, and they are also used extensively for curing coatings on furniture, automobile and truck bodies.
One disadvantage of this, my prior art Radiant-Wall Oven, is that in a pure heat transfer environment by radiation, the absorbing body (object) will continue to increase in temperature with time until its surface temperature approaches that of the emitting surface. Also, since infrared radiation is in the electromagnetic spectrum, it behaves exactly as light. Depending upon the shape of the part, object or substrate containing the coating to be dried, it would be most difficult to achieve absolute uniform surface temperatures in a pure radiant heat transfer system, simply because it would be difficult for each portion of the surface to receive an equal amount of incident radiant energy. When curing a coating on the interior surface of a vehicle, the problem of transferring the heat uniformly becomes even more complex and difficult. However, it should be noted that, in a radiant heat transfer system in which the shape factor is very nearly 1, uniform heat distribution can be maintained on the primary surfaces of a vehicle or part. Therefore, it is possible to achieve complete curing of exterior coatings on such objects using low intensity radiation, only.
Included in the prior art is an oven developed by me which combined into the Radiant-Wall Oven of U.S. Pat. No. 4,546,553, fans along the roof of the oven. This prior art oven had no conveyor system and was used for in situ drying of the paint on individual automobiles whose bodies had been lengthened or otherwise modified that would require them to be repainted. The process included disposing a single automobile with wet paint in the oven in a stationary position beneath the fans. Therefore, the paint was dried to a tack-free condition using only the radiant heat generated by the curved walls and the fans then came on to provide the final cure. In this oven design, the hot gases that are used to first heat the emitting wall are discharged into the oven cavity and there is not a means of providing simultaneous control of the emitter surface temperature and the air temperature. In fact, in this type of oven there is no direct control of the internal ambient temperature and the final curing conditions are determined from trial and error.
The development of the oven of the present invention provides an apparatus and method by which highly efficient heat transfer by infrared radiation is used while, at the same time, the equilibrium temperature of the surface of an object in the oven is controlled and the variation of temperature distribution is minimized through the use of air movement within the controlled chamber. This is accomplished, in the present invention, by applying air circulation over the radiant heat transfer surface and circulating the air at a lower temperature within the oven enclosure.
I have found that, using the present invention, the ambient air contained within the radiant heat transfer environment of the oven can be substantially lower in temperature than the emitter walls and the desired surface temperature of the vehicle body, part, object or substrate. Not only can the ambient temperature be lower, but for known emitter and ambient temperatures the equilibrium surface temperature of an object such as a freshly painted vehicle body can be very accurately predicted by a numerical method for digital computation. Thus, the oven design of the present invention provides the flexibility for controlling the levels of radiation and the ambient temperatures simultaneously. The combination of controlling these two heat transfer modes creates a multitude of heat transfer conditions. This feature is extremely beneficial, considering the vast number of curing cycles which now exist, and provides the flexibility for curing future coatings, especially water based and powder types of coatings.
In the oven of the present invention, the simultaneous control of the radiant energy emission and the ambient temperature of the air surrounding the processed object surfaces provides a family of heat transfer conditions that will ensure an exact and predictable equilibrium surface temperature of an object. Since a combination of the exchange of energy is created by the use of radiant energy and convection, the desired surface temperature of the object can be achieved by transferring most of the energy through radiation, therefore allowing the ambient temperature within the heat transfer environment to be less than the desired surface temperature of the object. In fact, as the radiant energy is absorbed by the object's surfaces that increase their temperature at a level higher than the surrounding ambient temperature, energy is exchanged between these surfaces and the ambient air. In other words, the air surrounding the object surfaces then starts a cooling process of the surfaces creating a final stabilized surface temperature that is in equilibrium with the radiant energy absorbed and the energy given up to the atmosphere of the oven by convection. This phenomena is explained by the following derivation of the equations that combine the heat transfer modes of radiation and convection. ##EQU1##
The solution to the equation is graphically demonstrated in FIGS. 11, 12, and 13. FIGS. 11, 12 and 13 demonstrate that for a known and controlled ambient temperature along with a known and controlled emitter temperature that an object surface temperature is achieved and maintained accurately and predictably at a constant amount. In other words, a specific advantage of this type of oven is that the part temperature remains uniform after it has reached its equilibrium temperature regardless of the exposure time. In a pure radiant oven, if the time cycle was extended for instance due to a conveyor stoppage, the surface temperatures would rise until the final equilibrium temperature would be that of the walls themselves. In a convection oven, forced or free, in the event the processed time is extended the part continuously increases in temperature until it approaches the air temperature contained within the oven. Since in an oven of the present invention, the ambient temperature can be considerably lower than the desired object surface temperature, a final equilibrium temperature can exist that will remain constant independent of exposure time. This obvious benefit results from the total energy exchange within the oven environment being in equilibrium.
FIG. 12 is a curve that demonstrates the family of conditions between the emitter temperature and the ambient temperature which will provide for a constant object surface temperature. As an example, if a part temperature of 200.degree. F. was desired, it could be achieved by using an emitting temperature of 200.degree. F. and ambient temperature of 200.degree. F. The exact same condition could also be created by using an emitter temperature of 300.degree. F. and an ambient temperature of approximately 100.degree. F. The advantage of being able to maintain a constant surface temperature of an object becomes apparent when the object processed is a vehicle possibly containing plastic and/or glass parts. In many instances, these types of parts are deformed from excessive heat if there is a line stoppage during the curing cycle. In an oven of this invention, the final equilibrium temperature that would prevent damage to various surfaces can be predicted and maintained.
In the oven of the present invention, millions of therms of energy can be saved because of the large decrease in energy level of the exhaust gases. In a conventional oven, to achieve a part temperature of, for example, 325.degree. F., the oven would normally be operated at least at a temperature of 350.degree. F. Therefore, all of the exhaust gases would probably be discharged at the higher energy level. In my present oven, the same desired surface temperatures can be achieved with an ambient temperature, much lower.
In a conventional 150 ft. oven the air exhaust rate would probably be 6,000 CFM. If the temperature of the exhaust gases were lowered 150.degree. F., by using the oven of the present invention almost 1,000,000 Btu/Hr. would be saved. When the energy saved from one oven is translated into the energy that could be saved for an entire finishing system, the total becomes very impressive.
Another source of energy savings in the oven of this invention is due to the fact that the end losses to, i.e., heat losses through the entrance and exit ends of the oven, are greatly decreased due to the lower operating ambient temperature in the oven environment. In a conventional oven operating at 350.degree. F. where air seals are used on the ends of the oven, the end losses can be as much as 800,000 Btu/Hr. depending upon the oven height. Contrary to popular belief, air seals, in most instances, increase the heat loss from an oven as opposed to decreasing it. The air seals decrease the temperature of the air that may escape from the end openings by dilution but usually the heat loss from the oven is increased due to the air movement at the interface of the oven with the ambient conditions exterior to the oven. No air seals are required on my present oven simply because the oven can be operated at a much lower ambient temperature and the infrared radiation can be contained in the central heating chamber.
The theory of the present invention is that most solids are opaque to nearly all thermal radiation, and therefore the emission (or absorption) of radiation takes place within a very thin surface layer (usually less than 0.0003" of the exterior of the surface). A notable exception to this is glass, which, although a solid, is transparent to short wave length thermal radiation (light) but is opaque to longer wave length radiation emitted by bodies at any temperature lower than that required to produce light. Liquids and gases, as well as some other solids are transparent to a greater or lesser degree. However, the primary concern is the absorption of radiation in a liquid coating. Experiments under my direction have shown that most coatings absorb all of the incident radiation within an extremely thin surface layer. Heat, transferred in the form of convection or conduction, requires a physical medium for transporting the energy from a high temperature source to a low temperature sink. Since radiant heat transfer can take place in a vacuum, it is evident that no heat transporting medium is required to transfer energy by infrared radiation. Unlike convection heat transfer where the energy is actually imparted to the surface from a physical medium, when a body absorbs infrared radiation the heat is actually generated within a very thin layer of the absorbing surface. It is widely accepted that the heat generated is due to a random motion imparted to the atoms and molecules that have absorbed the incident radiant energy.
In my present oven, I heat by using infrared radiation and using convection as a stabilizer. Thus, my present oven provides a heat transfer environment in which the ambient temperature does not have to be greater than the desired part temperature and in many instances can be considerably less. In most conventional convection ovens the operating temperature has to be substantially greater than the desired part surface temperature.
In the oven of the present invention the ambient temperature in the central heating chamber or environment of the oven can be several hundred degrees less than the desired part temperature and the ambient air actually acts to cool the surface of an object as opposed to heating the surface. A key element to the thermal efficiency of my present oven is the amount of energy that is consumed and not the operating temperature.
My oven is designed to include large radiating walls or surfaces directly opposed from one another. The remaining surfaces, i.e., top and bottom surfaces within the oven environment are usually reflective and have low emissivities. Therefore, all of the internal cavity surfaces of the oven are either emitting surfaces or reflective surfaces. The transmission losses from the heat generating source behind the radiant walls and the reflective surfaces is usually negligible due to the insulated panels behind these walls, so that the total energy consumed is essentially independent of the emitting surface temperature. In other words, since both emitting surfaces are at the exact same temperature, the exchange of energy between these two surfaces is equal. Since in my oven most of the radiation that falls upon a reflector will eventually be directed back onto the emitting surface, then the radiant exchange within the oven is in equilibrium and no appreciable radiant energy escapes the oven enclosure. This would be the case if the emitter surface temperatures were, for example, operating at 300.degree. F. or 800.degree. F. Therefore, only when an external body, at a lower temperature than the walls, is placed between the walls does any appreciable exchange of energy occur. Furthermore, essentially only that amount of energy that is absorbed by the external body is transferred from the system. All other radiant energy continues to be interchanged equally within the oven enclosure.
Air within the oven environment is maintained at a lower temperature than the desired part or object surface temperature and obviously is maintained at a temperature less than the emitting surface temperature. Those parts or objects in the central chamber will stabilize at a precise temperature above the ambient temperature and below the emitter surface temperature. However, it is important to recognize that here again, the total energy consumed is only that energy that is transferred to the external part. Therefore, the energy required for the part or object to reach the desired curing temperature is essentially independent of the ambient temperature.
Accordingly, it is an object of the present invention to provide an oven and process for rapidly and more uniformly drying paint on freshly painted surfaces on successive objects.
Another object of the present invention is to provide an oven for drying coatings on successive objects which oven is inexpensive to manufacture, durable in structure and efficient in operation.
Another object of the present invention is to provide an oven in which rapid drying of coatings on successive objects can be achieved at substantially lower oven air temperatures.
Another object of the present invention is to provide an oven and process for drying coatings on objects in which a clean essentially dust free environment is provided.
Another object of the present invention is to provide an oven for drying coatings on successive objects, the oven operating at lower air temperatures than conventional ovens.
Another object of the present invention is to provide an oven and process for drying coatings on successive objects using a minimum amount of heat.
Another object of the present invention is to provide an apparatus and process for automatically drying coatings on successive objects in which the exchange of heat between the oven and the ambient air in a plant is minimized.
Another object of the present invention is to provide an oven and process for drying coatings on objects, which will maximize the heat transfer and fuel efficiency of the oven.
Another object of the present invention is to provide an oven and a process for drying coatings on objects, which will reduce to a minimum the contamination from particulate matter and products of combustion.
Another object of the present invention is to provide an oven for drying coatings on objects in which the amount of energy transferred by infrared radiation and convection can be independently and simultaneously controlled.
Another object of the present invention is to provide an oven in which the temperature of the exhaust gas is substantially less than in other convective type ovens.
Another object of the present invention is to provide an oven which is modular and can be readily and easily expanded so as to provide different drying conditions when the assembly line in which the oven operates is modified.
Another object of the present invention is to provide an oven which is readily and easily shipped in a prefabricated condition and can be installed by semi-skilled laborers.
Another object of the present invention is to provide an oven which will provide essentially no adverse effects on the color of the coating which is being dried.
Another object of the present invention is to provide an oven which can be readily and easily cleaned.
Another object of the present invention is to provide an oven which can maintain, within very small tolerances, the equilibrium temperatures on the exterior surface of objects which pass through the oven.
Another object of the present invention is to provide an oven that will maintain a constant equilibrium temperature on the surface of objects, independent of time, to insure that sensitive coatings or materials will not be harmed if there is an interruption of the conveyance means.
Another object of the present invention is to provide an oven which be readily adapted to accommodate future coatings, such as water based and powder type paints.
Another object of the present invention is to provide an oven in which there is no need for air seals at the ends of the ovens.
Other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings wherein like characters of reference designate corresponding parts throughout the several views.